Numerous people work in noisy environments in which they are exposed to very high levels of noise coming from a variety of sources. Such noisy environments are to be found for example on building sites, on demolition sites (dynamiting), in factories using machine saws, etc. In such noisy environments, it is necessary to reduce noise levels in order to protect the hearing of such people.
Other people work in noisy environments in which noise levels exist in continuous manner that are lower, thereby presenting a smaller risk in the short term of deteriorating hearing capacity, but tending to increase fatigue and stress, and to reduce the capacities of such people for attention and concentration. Such noisy environments are to be found for example in open work spaces as used in call centers. The level of noise needs to be attenuated in order to improve the working conditions of such people.
Other people, in a non-professional setting, also desire to reduce the levels of noise in their environment in order to improve their own comfort, e.g. when they seek to rest, to concentrate (in order to read), or indeed when they desire to benefit fully from music they are listening to.
In most such situations, it is important to reduce noise levels effectively, but it can also be most advantageous to avoid complete acoustic isolation of the person in question so as to allow that person, while still being effectively protected against undesirable noise, to perceive useful noises: voices, alarms, doorbells, etc. In other words, it is very advantageous to adapt the level of noise attenuation as a function of the type of the noise, and possibly also as a function of the level of the noise, of the type of outside environment, etc.
Certain hearing protection devices are provided with noise control systems that make it possible to attenuate noise in different manners depending on the level of the noise.
For example, headsets are known that are dedicated to providing protection against the loud noises produced by an impact or an explosion. Such headsets are used mainly to attenuate the noise of shooting guns. They limit the noise of shots while still allowing the user to hear sounds of lower level. Such headsets include two earpieces, each generally comprising a shell, a microphone, and a loudspeaker. The shell comprises material of high density that gives a very large amount of passive attenuation. The microphone picks up surrounding noise outside the shell and plays back a filtered signal so that sounds that do not exceed a certain predefined threshold (typically 88 A-weighted decibels (dB(A))) are transmitted to a loudspeaker that then plays them back to the user's ear. By way of example, the user can thus hear voice sounds, while the sounds of shots are smothered.
Intra-auricular earphones are also known that operate in similar manner to the above-described headsets and that are particularly adapted to protecting a user against loud noises such as those encountered during building work or in particularly noisy industrial environments (e.g. see Document U.S. Pat. No. 5,355,418).
Such headsets and earphones thus adapt the way noise is controlled to the level of external noise, but not to the type of external noise. They therefore do not make it possible to attenuate an undesirable noise while amplifying a useful noise in a noisy environment in circumstances where the useful noise and the undesirable noise present equivalent sound levels.